Ping.fm had automatic features for hashtagging on Twitter without posting the tags to the likes of Facebook. You could either just hashtag individual words, and it'd strip them from non-hashtag compatible services, or you could add "@t tag1, tag2" to the end of your post, and Ping.fm would find those words in your post and tag them, and if they didn't exist, add them to the end of the post.

For example, as post with the following:
Check out this #kickass awesome link!: http://blah.com @t awesome, funny

Would result in the following on Twitter:
Check out this #kickass #awesome link!: http://blah.com #funny

And appear as the following on Facebook:
Check out this kickass awesome link!: http://blah.com

It was easy to use and made hashtagging possible across networks. It'd be nice to see it back in play.